Answer:
A. The cast-steel plow was better suited to Illinois soil.
Explanation:
The best that explains the success of Deere's invention is "the cast-steel plow was better suited to Illinois soil".
In an Illinois sawmill, John Deere, a young blacksmith saw a long, steel saw blade that was broken and he took it home. At that time, steel was a scarce commodity. When Deere took it home, he forged the blade into a plow. It is said that this plow contributed to the farming revolution. The plow was very much suited to be used in Illinois soil.